Plate type heat exchanger and method for manufacture thereof

ABSTRACT

A plate type heat exchanger wherein contacting portions of laminated plural plates and fins or contacting portions of laminated plural plates are brazed to form a heat exchange area, characterized in that at least the surface of a plate or fin contacting with a fluid is covered with an alloy comprising in weight ratio 25–35% of chromium, 5–7% of phosphorus, 3–5% of silicon, 0.001–0.1% of at least one selected from the group consisting of aluminum, calcium, yttrium and mischmetal, and balance containing mainly nickel. The alloy may contain 15% or less of iron and or 10% or less of molybdenum. 
     The plate type exchanger exhibits enhanced pressure resistance and is excellent in corrosion resistance.

TECHNICAL FIELD

The present invention relates to a plate type heat exchanger which has aheat exchanging portion constituted by laminating plural plates withfins or a plural of plates and brazing them mutually, and a method forits preparation.

BACKGROUND ART

In a plate type heat exchanger, the heat exchanging portion isconstituted, for example, by laminating alternately plates and fins andbrazing the portions where each plates contact with fins. Also, it ispossible to form uneven portions for stirring on the flat surface of aplate and thus to omit the fins. In this case, the contacting portion ofeach plates is brazed.

As the brazing filler metal used for brazing, brazing copper alloy whichis rich in practical application in various fields has been widely used.However, in heat exchangers requiring high temperature characteristicsand excellent corrosion resistance, brazing nickel alloy superior inthese features to brazing copper alloy in general has come to be alsoused.

As brazing nickel alloy is poor in processability, when brazing nickelalloy in powder form is applied for a plate type heat exchanger, thebrazing alloy powder is adhered on the brazing parts such as the convexportions or flange portions formed on the plate or on the plate surfacecontacting with the fins and it is heated to braze.

Heretofore, as methods for adhering the brazing alloy powder on such aplate surface, the following methods have been known; that a pasteprepared by mixing previously a brazing filler metal with a liquidbinder for fixing the brazing filler metal is adhered by screenprinting, that a paste brazing filler metal is adhered by using adispenser, and that a brazing filler metal and a binder are sprayedsimultaneously and adhered. Also, as the binder, an aqueous binder hasbeen widely used in consideration of safety and environmental problems.

Recently, a heat exchanger excellent in corrosion resistance andstrength characteristics has come to be required for further improvementon reliability and hence a brazing nickel alloy excellent in corrosionresistance and high in brazing strength has come to be required tosatisfy the requirement.

However, the nickel-chromium-boron-silicon alloy (for example, JIS Z3265 BNi-2) widely used as the brazing nickel alloy has a problem thatit is poor in corrosion resistance though high in brazing strength. Asit is also poor in fluidity, corrosion in the vicinity of the boundarybetween the brazing filler metal and the parts to be brazed had come tobe a problem.

JP 2000-171188 A discloses a plate type heat exchanger brazed with abrazing nickel-chromium-phosphorus alloy excellent in fluidity isdisclosed. Also, in JP 2000-190069 A, a method for the preparation of aplate type heat exchanger by using a brazing nickel-chromium-phosphorusalloy is disclosed. However, BNi-7 prescribed in JIS standard as atypical brazing nickel-chromium-phosphorus alloy had problems that itwas lower in brazing strength and poorer in corrosion resistance thanother brazing nickel alloys.

An object of the present invention is to provide a plate type heatexchanger high in pressure resistance and excellent in corrosionresistance and a method for its preparation.

DISCLOSURE OF THE INVENTION

The plate type heat exchanger according to the present invention, inwhich contacting portions of laminated plural plates and fins orcontacting portions of laminated plural plates are brazed to form a heatexchange area, is characterized in that at least the plate surface andthe fin surface contacting to the fluid are covered by an alloycomprising in weight ration 25 to 35% chromium, 5 to 7% phosphorus, 3 to5% silicon, 0.001 to 0.1% of at least one selected from the groupconsisting of aluminum, calcium, yttrium and mischmetal and balance ofmainly nickel. The alloy may contain not more than 15% of iron and/ornot more than 10% of molybdenum.

Also, a method for the preparation of the plate type heat exchangeraccording to the present invention high in workability is characterizedin that a binder is applied by a spray coating on both or one of theplate and the fin and a brazing alloy powder consisting of the alloymentioned above is adhered on said surface on which the binder wasapplied and then a plural of them is laminated and brazed.

When the nickel alloy according to the present invention is used as thebrazing filler metal, a plate type heat exchanger of high pressureresistance can be obtained as it is high in brazing strength. Also, asit is excellent in fluidity, when it is melted during brazing, it flowson the surface of plates and fins constituting the heat exchangingportion and thus it can cover the plate surface and the fin surfacecontacting to the fluid. As the result, in the vicinity of the front ofthe flow of the brazing filler metal, corrosion caused by the corrosionpotential difference between the brazing filler metal and the plates orthe fins can be prevented.

Accordingly, the plate type heat exchanger according to the presentinvention can be applied for corrosive fluids, because the surfaces ofplates and fins contacting to a fluid feared to corrode them are coveredby the nickel alloy excellent in corrosion resistance.

Now, the reason for limiting the amounts of each components contained inthe nickel alloy according to the present invention will be illustratedas follows.

Chromium is an element which forms a nickel-chromium solid solution bybeing dissolved in nickel and is necessary for improving corrosionresistance and oxidation resistance. However, a content lower than 25%gives little effect on the improvement of sulfuric acid resistance andstrength, while a higher content lowers wettability. Therefore, it ismade to be 25 to 35%.

Phosphorus in an amount lower than 5% raises the melting point of thealloy, while that in an amount higher than 7% deteriorates the corrosionresistance and lowers the strength. Hence, it is made to be 5 to 7%.

Silicon in an amount lower than 3% deteriorates the corrosion resistanceand lowers the strength, while that in an amount higher than 5% raisesthe melting point. Therefore, it is made to be 3 to 5%.

Each of molybdenum and iron is an element effective for improving thestrength of the alloy. However, higher contents give little effect forthe improvement of strength and also deteriorate corrosion resistance.Hence, they are made to be respectively not more than 10% and not morethan 15%.

Aluminum, calcium, yttrium and mischmetal are elements necessary forimproving wettability. However, an amount lower than 0.001% gives littleeffect, while that higher than 0.1% lowers wettability and strength.Therefore, it is made to be 0.001 to 0.1%. Use of aluminum isparticularly preferred.

Furthermore, in the nickel alloy used in the present invention, part ofnickel may be replaced by cobalt. In this case, it is preferred that theused amount of cobalt is not higher than 10% of the alloy.

In the present invention, as the binder for adhering the brazing alloypowder to the plates and so, either an organic solvent type binder or anaqueous type binder can be used. However, it is preferred to use theaqueous type binder excellent in safety. Conventionally, the aqueoustype binder has been generally poor in wettability and low in adhesionand therefore it has been difficult to adhere a brazing alloy powdersurely on the part to be brazed. However, according to the presentinvention, the brazing filler metal can be fixed uniformly and surely byspray-coating the binder and then applying the brazing alloy powder onthe surface coated by the binder in a method of scattering or the like.Resultantly, a rapid treatment is possible.

As the aqueous type binders, there are exemplified aqueous emulsions andaqueous solutions of vinyl acetate resins, polyvinyl alcohols,polyvinylpyrrolidones, polyacrylamides, polyethylene oxides,polyethylene glycols and the like.

In the present invention, to get excellent corrosion resistance andfluidity, it is preferred that the brazing is carried out in anatmosphere of vacuum lower than 10⁻³ torr or a nonoxidative atmosphere.

Also, when corrosion becomes a problem only in one of the fluids flowingin the heat exchanger, the surface of the plates and fins may be coveredby the nickel alloy according to the present invention only on the sideof the path in which said fluid flows.

The present invention can be applied to a heat exchanging portionconstituted by laminating alternately plates and fins and brazingbetween the contacting portions of each plates and fins or for a heatexchanger constituted by forming spacers and uneven portions forstirring on the plate surface to omit the fins and brazing between thecontacting portions of plates or for a heat exchanger constituted byinserting the fins for only one of the fluids and brazing between thecontacting portions.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view of the general construction showing the conditions ofplates and fins in a plate type heat exchanger according to the presentinvention.

BEST EMBODIMENT FOR EXECUTING THE INVENTION

Examples of the present invention will be illustrated as follows.

In Examples, as shown in FIG. 1, a plate type heat exchanger constitutedby laminating alternately plates and fins and brazing them was preparedby a method in which a brazing alloy powder was adhered to plates 1 andthe plates were laminated with fins 2 and bars 3 to form a prescribedshape and then they were brazed.

EXAMPLE 1

An alloy A consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, a binder (aqueous polyvinyl alcohol solution) was sprayed on theboth surfaces of plates 1 made of 304 stainless steel and then thebrazing alloy powder prepared above was adhered on the binder and thensuccessively these plates 1 were laminated with fins 2 and bars 3 andbrazed in a vacuum of 10⁻⁴ torr at 1100° C. for 10 minutes to complete aplate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 63 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

The alloy A was melted and molded in a shell mold and a test piece of10×10×5 mm was prepared from it. The test piece was immersed in a 5%sulfuric acid solution to carry out a corrosion test. As the result, thecorrosion loss was 0.0001 mg/m²s.

EXAMPLE 2

An alloy B consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1, the brazing alloy powder wasadhered on plates made of 304 stainless steel and then they werelaminated with fins and bars and brazed in a vacuum of 10⁻⁴ torr at1090° C. for 10 minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 59 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was checked. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

A corrosion test was carried out on the alloy B in the same manner as inExample 1. As the result, the corrosion loss was 0.0003 mg/m²s.

EXAMPLE 3

An alloy C consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1 except that a polyvinyl acetateemulsion was used as the binder, the brazing alloy powder was adhered onplates made of 304 stainless steel and then they were laminated withfins and bars and brazed in a vacuum of 10⁻⁴ torr at 1080° C. for 10minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 61 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

A corrosion test was carried out on the alloy C in the same manner as inExample 1. As the result, the corrosion loss was 0.0002 mg/m²s.

EXAMPLE 4

An alloy D consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1 except that a polyvinyl acetateemulsion was used as the binder, the brazing alloy powder was adhered onplates made of 304 stainless steel and then they were laminated withfins and bars and brazed in a vacuum of 10⁻⁴ torr at 1080° C. for 10minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 58 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

A corrosion test was carried out on the alloy D in the same manner as inExample 1. As the result, the corrosion loss was 0.0003 mg/m²s.

EXAMPLE 5

An alloy E consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1 except that a polyvinyl acetateemulsion was used as the binder, the brazing alloy powder was adhered onplates made of 304 stainless steel and then they were laminated withfins and bars and brazed in a vacuum of 10⁻⁴ torr at 1090° C. for 10minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 60 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

A corrosion test was carried out on the alloy E in the same manner as inExample 1. As the result, the corrosion loss was 0.0004 mg/m²s.

EXAMPLE 6

An alloy F consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1 except that an aqueouspolyvinylpyrrolidon solution was used as the binder, the brazing alloypowder was adhered on plates made of 304 stainless steel and then theywere laminated with fins and bars and brazed in a vacuum of 10⁻⁴ torr at1090° C. for 10 minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 59 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

A corrosion test was carried out on the alloy F in the same manner as inExample 1. As the result, the corrosion loss was 0.0003 mg/m²s.

COMPARATIVE EXAMPLE 1

An alloy G consisting of the components shown in Table 1 was melted anda brazing alloy powder was prepared by a gas atomizing method.

Then, in the same manner as in Example 1 except that an aqueouspolyvinylpyrrolidon solution was used as the binder, the brazing alloypowder was adhered on plates made of 304 stainless steel and then theywere laminated with fins and bars and brazed in a vacuum of 10⁻⁴ torr at1080° C. for 10 minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 34 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

The alloy G was melted and a corrosion test was carried out on it in thesame manner as in Example 1. As the result, the corrosion loss was 0.002mg/m² s.

COMPARATIVE EXAMPLE 2

An alloy BNi-7 was melted and a brazing alloy powder was prepared by agas atomizing method.

Then, in the same manner as in Example 1, the brazing alloy powder wasadhered on plates made of 304 stainless steel and then they werelaminated with fins and bars and brazed in a vacuum of 10⁻⁴ torr at1080° C. for 10 minutes to complete a plate type heat exchanger.

The completed heat exchanger was subjected to a burst test. The pressureresistance was 31 kg/cm². After the burst test, the heat exchanger wascut and the flow of the brazing filler metal was examined. It was foundthat the surfaces of the plates and the fins were coated with thebrazing filler metal.

The alloy BNi-7 was melted and a corrosion test was carried out on it inthe same manner as in Example 1. As the result, the corrosion loss was0.015 mg/m²s.

The test results of Examples 1 to 7 and Comparative Examples 1 and 2 areshown in Table 2 together with the materials used.

TABLE 1 Component ratio (weight %) Cr P Si Al Ca Mo Fe Co Ni Alloy A 296 4.5 0.03 — 3   7 — Balance Alloy B 28 6 4 — 0.02 5 — — Balance Alloy C29 6.1 4.3 0.02 — — — — Balance Alloy D 30 6.2 4.5 0.03 — — 8.1 —Balance Alloy E 29 5.8 4.5 0.02 — — 1.2 8 Balance Alloy F 28 6 4 0.0020.02 5 0.1 — Balance Alloy G 24 9.5 — — — — — — Balance BNi-7 13 10 — —— — — — Balance

TABLE 2 Test result Pressure Corrosion Materials used resistance lossAlloy Binder (kg/cm²) (mg/m²s) Example 1 Alloy A Polyvinyl alcohol 630.0001 Example 2 Alloy B Polyvinyl alcohol 59 0.0003 Example 3 Alloy CPolyvinyl acetate 61 0.0002 Example 4 Alloy D Polyvinyl acetate 580.0003 Example 5 Alloy E Polyvinyl acetate 60 0.0004 Example 6 Alloy FPolyvinyl pyrrolidone 59 0.0003 Comp. Ex. 1 Alloy G Polyvinylpyrrolidone 34 0.002 Comp. Ex. 2 BNi-7 Polyvinyl alcohol 31 0.015

As shown above, the plate type heat exchangers prepared by Examplesaccording to the present invention have come to be highly excellent inpressure resistance and corrosion resistance.

The indication % means weight % in the present specification unlessspecified.

1. A plate-type heat exchanger having laminated plural plates withbrazed contacting portions to form a heat exchange area, comprising: atleast a surface of a plate in contact with a fluid and covered with analloy comprising 25 to 35 wt. % of chromium, 5 to 7 wt. % of phosphorus,3 to 5 wt. % of silicon, 0.001 to 0.1 wt. % of aluminum and the balancecontaining mainly nickel.
 2. The plate-type heat exchanger according toclaim 1, in which said alloy contains iron in an amount not higher than15%.
 3. The plate-type heat exchanger according to claim 1, in whichsaid alloy contains molybdenum in an amount not higher than 10%.
 4. Amethod of preparing a plate-type heat exchanger having laminated pluralplates and fins to form a heat exchange area, comprising the steps of:applying a binder on at least one of a plate and a fin by spray-coating;then adhering a brazing alloy powder consisting of a nickel alloy on asurface applied with the binder; and then repetitively laminating andheating to effect brazing, said brazing alloy comprising 25 to 35 wt. %of chromium, 5 to 7 wt. % of phosphorus, 3 to 5 wt. % of silicon, 0.001to 0.1 wt. % of aluminum and the balance containing mainly nickel.
 5. Aplate-type heat exchanger having laminated plural plates with brazedcontacting portions to form a heat exchange area, comprising: at least asurface of a plate in contact with a fluid and covered with an alloycomprising 25 to 35 wt. % of chromium, 5 to 7 wt. % of phosphorus, 3 to5 wt. % of silicon, 0.001 to 0.1 wt. % of calcium and the balancecontaining mainly nickel.
 6. The plate-type heat exchanger according toclaim 5, in which said alloy contains iron in an amount not higher than15%.
 7. The plate-type heat exchanger according to claim 5, in whichsaid alloy contains molybdenum in an amount not higher than 10%.
 8. Amethod of preparing a plate-type heat exchanger having laminated pluralplates and fins to form a heat exchange area, comprising the steps of:applying a binder on at least one of a plate and a fin by spray-coating;then adhering a brazing alloy powder consisting of a nickel alloy on asurface applied with the binder; and then repetitively laminating andheating to effect brazing, said brazing alloy comprising 25 to 35 wt. %of chromium, 5 to 7 wt. % of phosphorus, 3 to 5 wt. % of silicon, 0.001to 0.1 wt. % of calcium and the balance containing mainly nickel.